ECG Axis Interpretation

The diagram below illustrates the relationship between QRS axis and the frontal leads of the ECG.

Hexaxial Reference system

Image reproduced from Chung

 

  • Normal Axis = QRS axis between -30 and +90 degrees.
  • Left Axis Deviation = QRS axis less than -30 degrees.
  • Right Axis Deviation = QRS axis greater than +90 degrees.
  • Extreme Axis Deviation = QRS axis between -90 and 180 degrees (AKA “Northwest Axis”).

There are several complementary approaches to estimating QRS axis, which are summarised below.

Method 1 – The Quadrant Method

The most efficient way to estimate axis is to look at leads I + aVF.

Lead I
Lead aVF
Quadrant
Axis
PositivePositiveLeft lower quadrantNormal (0 to +90 degrees)
PositiveNegativeLeft upper quadrantPossible LAD (0 to -90 degrees)
NegativePositiveRight lower quadrantRAD (+90 to 180 degrees)
NegativeNegativeRight upper quadrantExtreme Axis Deviation (-90 to 180 degrees)

 

Method 2 – Leads I + II

Another rapid method is to look at leads I + II.

A positive QRS in lead I puts the axis in roughly the same direction as lead I.

lead I

Image reproduced from Chung

 

A positive QRS in lead II similarly aligns the axis with lead II.

lead II

Image reproduced from Chung

 

Therefore, if leads I and II are both positive, the axis is between -30 and +90 degrees (i.e. normal axis).

normal axis I and II

Image reproduced from Chung

 

Combining Methods 1 and 2

By combining these two methods, you can rapidly and accurately assess axis.

Lead I
Lead aVF
Axis
PositivePositiveNormal (0 to +90 degrees)
PositiveNegativePossible LAD
Is lead II positive?
Yes -> Normal (0 to -30 degrees)
No -> LAD (-30 to -90 degrees)
NegativePositiveRAD (+90 to 180 degrees)
NegativeNegativeExtreme Axis Deviation (-90 to 180 degrees)

 

Method 3 – The Isoelectric Lead

This method allows a more precise estimation of QRS axis, using the axis diagram below.

Hexaxial Reference system

Reproduced from Chung

 

Key Principles

  • If the QRS is positive in any given lead, the axis points in roughly the same direction as this lead.
  • If the QRS is negative in any given lead, the axis points in roughly the opposite direction to this lead.
  • If the QRS is isoelectric in any given lead (positive deflection = negative deflection), the axis is at 90 degrees to this lead.

Step 1. Find the isoelectric lead.

The isoelectric (equiphasic) lead is the frontal lead with zero net amplitude. This can be either:

  • A biphasic QRS where R wave height = Q or S wave depth.
  • A flat-line QRS with no discernible features.

Step 2. Find the positive leads. 

Look for the leads with the tallest R waves (or largest R/S ratios).

Step 3. Calculate the QRS axis. 

The QRS axis is at 90 degrees to the isoelectric lead, pointing in the direction of the positive leads.

This concept can be difficult to understand at first, and is best illustrated by some examples.

 

Example 1

Axis eg1

  • Leads I + aVF are both positive.
  • This puts the axis in the left lower quadrant, between 0 and +90 degrees, i.e. normal axis.
  • Lead II is also positive, which confirms the normal axis.

  • Lead aVL is isoelectric, being biphasic with similarly sized positive and negative deflections (no need to precisely measure this).
  • From the diagram above, we can see that aVL is located at -30 degrees.
  • The QRS axis must be ± 90 degrees from lead aVL, either at +60 or -120 degrees.
  • With leads I (0), II (+60) and aVF (+90) all being positive, we know that the axis must lie somewhere between 0 and +90 degrees.
  • This puts the QRS axis at +60 degrees.

 

Example 2

Ex2

  • Lead I = negative.
  • Lead aVF = positive.
  • This puts the axis in the right lower quadrant, between +90 and +180 degrees, i.e. RAD.

  • Lead II (+60 degrees) is the isoelectric lead.
  • The QRS axis must be ± 90 degrees from lead II, at either +150 or -30 degrees.
  • The more rightward-facing leads III (+120) and aVF (+90) are positive, while aVL (-30) is negative.
  • This puts the QRS axis at +150 degrees.

This is an example of right axis deviation secondary to right ventricular hypertrophy.

 

Example 3

Ex3

  • Lead I = positive.
  • Lead aVF = negative.
  • This puts the axis in the left upper quadrant, between 0 and -90 degrees, i.e. normal or LAD.
  • Lead II is neither positive nor negative (isoelectric), indicating borderline LAD.

  • Lead II (+60 degrees) is isoelectric.
  • The QRS axis must be ± 90 degrees from lead II, at either +150 or -30 degrees.
  • The more leftward-facing leads I (0) and aVL (-30) are positive, while lead III (+120) is negative.
  • This confirms that the axis is at -30 degrees.

This is an example of borderline left axis deviation due to inferior MI.

 

Example 4

Ex4

  • Lead I = negative.
  • Lead aVF = negative.
  • This puts the axis in the upper right quadrant, between -90 and 180 degrees, i.e. extreme axis deviation.

NB. The presence of a positive QRS in aVR + negative QRS in multiple other leads is another clue to the presence of extreme axis deviation. 

  • The most isoelectric lead is aVL (-30 degrees).
  • The QRS axis must be at ± 90 degrees from aVL at either +60 or -120 degrees.
  • Lead aVR (-150) is positive, with lead II (+60) negative.
  • This puts the axis at -120 degrees.

This is an example of extreme axis deviation due to ventricular tachycardia.

 

Example 5

Ex5

  • Lead I = isoelectric.
  • Lead aVF = positive.
  • This is the easiest axis you will ever have to calculate. It has to be at right angles to lead I and in the direction of aVF, which makes it exactly +90 degrees!

This is referred to as a “vertical axis”  and is seen in patients with emphysema who typically have a vertically orientated heart.

EmphysemaCXR

Vertical Heart in Emphysema

 

Causes of Axis Deviation

Right Axis Deviation

 

Left Axis Deviation

 

Extreme Axis Deviation

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